packed_binaries: Extract binaries in parallel

Currently we extract each Gitaly's packed binaries serially. This task
consumes a majority of the time spent during initialization. By
extracting these files in parallel we can measureably reduce startup
time.

On a 4-core system this improves startup time by ~10%, using the same
benchmarking process as 96438c249 (gitaly: Don't block on preloading
licensedb, 2023-09-20):

  Benchmark 1: ./gitaly-par serve config.toml
    Time (mean ± σ):     228.5 ms ±   4.2 ms    [User: 285.8 ms, System: 76.8 ms]
    Range (min … max):   222.7 ms … 237.0 ms    13 runs

  Benchmark 2: ./gitaly-st serve config.toml
    Time (mean ± σ):     254.5 ms ±   6.9 ms    [User: 315.1 ms, System: 75.6 ms]
    Range (min … max):   246.7 ms … 272.2 ms    11 runs

  Summary
    ./gitaly-par serve config.toml ran
      1.11 ± 0.04 times faster than ./gitaly-st serve config.toml

On a 16-core system this improves to ~20%:

  Benchmark 1: ./gitaly-par serve config.toml
    Time (mean ± σ):     234.7 ms ±   6.0 ms    [User: 326.4 ms, System: 169.5 ms]
    Range (min … max):   228.6 ms … 247.5 ms    12 runs

  Benchmark 2: ./gitaly-st serve config.toml
    Time (mean ± σ):     282.9 ms ±  10.4 ms    [User: 377.1 ms, System: 156.3 ms]
    Range (min … max):   266.3 ms … 302.0 ms    11 runs

  Summary
    './gitaly-par serve config.toml' ran
      1.21 ± 0.05 times faster than './gitaly-st serve config.toml'

This does place more demand on the disk, but only momentarily. When the
host is under heavy io pressure, simulated here with `stress-ng --iomix
5` parallel extraction's performance advantage is extended to ~36% on
the 16-core system:

  Benchmark 1: ./gitaly-par serve config.toml
    Time (mean ± σ):     545.9 ms ± 159.9 ms    [User: 731.7 ms, System: 177.7 ms]
    Range (min … max):   326.9 ms … 913.7 ms    10 runs

  Benchmark 2: ./gitaly-st serve config.toml
    Time (mean ± σ):     740.9 ms ± 242.8 ms    [User: 977.4 ms, System: 150.7 ms]
    Range (min … max):   378.9 ms … 1029.0 ms    10 runs

  Summary
    './gitaly-par serve config.toml' ran
      1.36 ± 0.60 times faster than './gitaly-st serve config.toml'

Changelog: performance

1 files changed, 6 insertions, 6 deletions

diff --git a/packed_binaries.go b/packed_binaries.go
index f589282c0..e0ec37c0b 100644
--- a/packed_binaries.go
+++ b/packed_binaries.go
@@ -8,6 +8,7 @@ import (
 	"path/filepath"
 
 	"gitlab.com/gitlab-org/gitaly/v16/internal/helper/perm"
+	"golang.org/x/sync/errgroup"
 )
 
 // buildDir is the directory path where our build target places the built binaries.
@@ -35,8 +36,10 @@ func UnpackAuxiliaryBinaries(destinationDir string) error {
 		return fmt.Errorf("list packed binaries: %w", err)
 	}
 
+	g := &errgroup.Group{}
 	for _, entry := range entries {
-		if err := func() error {
+		entry := entry
+		g.Go(func() error {
 			packedPath := filepath.Join(buildDir, entry.Name())
 			packedFile, err := packedBinariesFS.Open(packedPath)
 			if err != nil {
@@ -70,10 +73,7 @@ func UnpackAuxiliaryBinaries(destinationDir string) error {
 			}
 
 			return nil
-		}(); err != nil {
-			return err
-		}
+		})
 	}
-
-	return nil
+	return g.Wait()
 }